1. Field of the Invention
The present invention relates generally to a flow control for a fluid separation system, and more particularly, to an adjustable flow control device, and preferably an externally adjustable flow control device, having particular applicability to a crossflow membrane separation system.
2. Description of the Prior Art
A fluid separation system of the type to which the present invention has particular applicability is a system which separates a feed solution into a concentrate or unfiltered stream and a permeate or filtered stream. Such a system typically includes a filter means such as a fluid separation membrane, a pump for pressurizing the feed solution, a concentrate stream facilitating flow of concentrate from the upstream side of the filter and a permeate stream facilitating removal of permeate from the downstream side of the filter. A valve or other restriction is commonly positioned within the concentrate stream to control the flow of concentrate from the system, and thus also the pressure of feed solution on the upstream side of the filter. Such a valve or restriction device may also be employed to control the flow of a portion of such concentrate, which is separated from the main concentrate stream and routed back or recycled upstream of the filter for combination with the feed stream. A crossflow separation system to which the present invention has particular applicability includes a crossflow membrane module such as a reverse osmosis module or the like, having a crossflow membrane across which pressurized feed solution is caused to flow.
There are several types of valves or restrictions which have been previously used in fluid separation systems for controlling flow of concentrate and/or recycle within the system. One such valve is a conventional needle valve positioned within the concentrate or recycle stream. A typical needle valve includes a circular valve seat through which concentrate is allowed to flow when the valve is in an open position and a conically shaped needle. The needle is adapted for movement into and out of engagement with the seat to control the flow of concentrate through the valve. In such needle valves, the seat which defines the flow orifice is positioned generally perpendicular to the axial direction of the stream. The advantage of a needle valve control is that it allows for virtually infinite control between a complete shutoff and a wide open flow. This wide range of flexibility, however, also results in its principal limitation. Because of the wide range of possible adjustments, and the extreme sensitivity of most fluid separation systems, adjustment of these needle control valves is usually handled by a skilled technician. This necessarily increases the cost of operation or, if a skilled technician is not employed, the chance of misfunction or malfunction of the system. A further limitation of a needle valve is the relative ease with which it may drift, either by vibration or the fluid flow against it, causing the precise setting required by the system to be altered.
Another practice is to simply drill a fixed orifice into a plate or other restriction positioned in the concentrate and/or recycle stream. This practice has the advantage of not requiring a skilled technician to operate the same, but has the obvious disadvantage of being relatively inflexible. Use of a fixed orifice has the further disadvantage in that the size of the orifice is seldom, if ever, right the first time for a particular system because of variables in the components and the accuracy of the orifice itself. This requires resizing of the orifice and/or the addition or closing of orifice holes and often requires multiple attempts to attain the proper flow rates and pressure. Additionally, if changes are needed in the field, a similar process must be repeated. A further disadvantage of a fixed orifice is the need for a separate flush valve to flush the system, when desired.
A further flow control mechanism for a fluid separation system involves a design which incorporates both a needle valve concept and a fixed orifice concept as shown in U.S. Pat. No. 3,716,141. This structure eliminates the need for a separate flush valve while also eliminating the need for adjustment by a skilled technician. Such structure, however, still has the disadvantages of a fixed orifice in that it does not permit adjustment of the orifice after installation, except by resizing the orifice, the addition or closing of orifice holes or the replacement of the orifice component.
Accordingly, there is a need for an improved flow control device for a fluid separation system and more specifically to an adjustable flow control mechanism for a fluid separation system which overcomes the limitations of the prior art.